A conventional VCO may include a ring oscillator that is formed of a controllable ring of inverter stages which utilize one or more internal resistors as load devices. Such a VCO may be utilized as an element of a PLL in a clock recovery circuit or other type of circuit. The free-running frequency of the ring oscillator strongly depends on the internal resistor value, at least in part because associated capacitance value does not vary as much as the resistor value.
A problem that arises in a conventional VCO of the type described above is that the internal resistor value is typically subject to an excessively large variation within a given integrated circuit manufacturing process. As a result, for a particular integrated circuit VCO design, integrated circuits from different fabrication lots of the same manufacturing process can have significant differences between the values of their respective internal resistors, and thus their achievable VCO oscillation frequencies. This problem adversely impacts the production yield of the integrated circuit manufacturing process, and increases device cost.
One approach to addressing this issue involves increasing the gain of the VCO, generally denoted KVCO, such that a higher oscillation frequency can be achieved for a given internal resistor value. However, this approach can cause a significant amount of additional jitter in the resultant output signal, which is clearly undesirable in many applications.
A need therefore exists in the art for techniques which permit the achievable VCO oscillation frequency to be accurately controlled from one integrated circuit device to another in the presence of process-related variations in internal resistor value, without requiring jitter-inducing increases in the VCO gain.